1. Field of the Invention
The invention is directed to a friction driven belted crawler having main drive wheels located at the rear of the crawler and auxiliary drive wheels located at the front of the crawler. The auxiliary drive wheels are driven at a percentage of the speed of the main drive wheels.
2. Description of the Prior Art
Conventional crawlers are well known and are typically provided with a steel segmented track consisting of steel track shoes that are mounted to a steel track chain. The track chain is positively driven by a drive sprocket which is coupled through a transmission to an internal combustion engine.
It has long been proposed to form endless belted tracks from elastomeric material. Such tracks have a number of advantages, one such advantage is that the belted tracks permit a crawler to drive over streets and curbs without ripping up streets or curbs with the steel grousers of conventional tracks. Two belted track systems are currently being used on construction and agricultural machines. In one design marketed by Bridgestone Co., Ltd., Tokyo, Japan, the track is provided with a series of apertures through which a toothed drive sprocket engages the track for positively driving the track. Each of the apertures is reinforced by a steel insert mounted in the track, see U.S. Pat. No. 4,678,244. The second system is currently being marketed by Caterpillar Tractor Co., Peoria, Ill., on an agricultural tractor, see PCT Application WO85/02824. With this track system, the belted track engages a rubber coated main drive wheel located at the rear of the crawler and a pneumatic tire idler wheel located at the front of the tractor. With a belted track system slippage may occur between the main drive wheel and the belted track. As such, the belted track is put under great tension to reduce slippage.
Friction driven belted tracks, such as that on the Caterpillar Tractor Co. agricultural tractor, do not provide comparable performance capabilities in the forward and reverse directions. In the forward direction, the minimum tension in the belt occurs on the top portion of the belted track, and the maximum tension occurs in the lower portion of the belt as it contacts the drive wheel. In the reverse direction, the opposite is true and the maximum tension occurs in the top portion of the belt, and the minimum tension occurs in the belt at its lower contact with the drive wheel. Therefore, the tractor has greater forward thrust than reverse thrust.
The maximum thrust potential of this track system is proportional to the track static pretension. Therefore, with a large heavy vehicle, it is necessary to keep the tracks highly tensioned to provide adequate reverse thrust. Higher pretension loading of the track necessitates increasing the frame structure of the track undercarriage to withstand high tension loadings.
One method of providing more comparable performance characteristics in both the forward and reverse directions is to drive all four wheels of the track. U.S. Pat. No. 4,560,018, to Satzler, discloses using a tandem drive mechanism to drive jointless elastomeric belt. This proposed mechanism delivers greater torque to the rear wheel assembly than the front wheel assembly.
Six wheel drive motor graders are being marketed by Deere & Company, Moline, Ill., the assignee of the present patent application, and Dresser Industries, Inc., Houston, Tex., wherein hydraulic wheels motors are used to drive the front wheels at a percentage of the speed of the tandem drive wheels, see U.S. Pat. Nos. 4,177,870, 4,183,419 and 4,186,816 to Deere & Company, and 4,546,844 and 4,635,793 to Dresser Industries. With these systems an electronic controller having wheel speed sensors automatically drives the front wheels at a preselected percentage of the speed of the rear main drive wheels. The front wheels are either driven in a normal mode at 97-99% of the speed of the main drive wheels; or in an aggressive mode at 101-103% of the speed of the main drive wheels. In the normal mode, the front wheels only become engaged when the rear wheels are slipping. In the aggressive mode, the front wheels are constantly trying to pull the machine.